1. Field of the Invention
The present invention relates a method and an apparatus for producing steels with predominantly high Cr content and/or ferro-alloys, wherein the metal charge is melted in a melting crucible, is alloyed, refined and reduced, and wherein process gases are simultaneously supplied through bottom nozzles.
2. Description of the Related Art
Various methods have already become known for manufacturing stain-resistant, acid-resistant and heat-resistant high-grade steels having a high degree of purity and low gas contents.
The best known method is melting in the arc furnace, or melting in the arc furnace with subsequent refining in a converter with the use of mixed gases or in a vacuum.
The manufacture of these stain-resistant, acid-resistant and heat-resistant high-grade steels only in the arc furnace is very complicated with respect to energy technology and is problematic with respect to the degree of purity to be adjusted.
The manufacture of the aforementioned steels in the arc furnace with subsequent refining in the previously known converters with the use of mixed gases has the disadvantage that these converters were originally designed for the exclusive manufacture of chromium steels which have a smaller specific reaction volume than is required for the manufacture of high-grade steel qualities with a wide spectrum of alloying contents. A requirement for the necessary high refining and heating speed is a large specific oxygen supply. In addition, during the deoxidation phase it is not possible to prevent a reoxidation of the melt by the air entering the reaction space. Moreover, an intensive air oxygen contact occurs during tapping of the reduced steel which results in harmful oxidic impurities and, thus, leads to waste in the product.
Generally used today for the above-described manufacture of high-alloy, high-grade steel, preferably with high Cr and Ni contents, are, as far as the plant concept is concerned, the so-called "duplex method", i.e., use of an electric arc furnace with a subsequent converter, as well as the so-called "triplex method", i.e., use of an arc furnace with subsequent converter and subsequent vacuum plant.
In both concepts of the plant, the arc furnace serves as a premelting unit. The actual refining process for manufacturing the desired steel quality with high alloy contents and low carbon contents takes place in a converter with lateral below-bath nozzles, in a converter with bottom nozzles, or in a converter with both nozzle arrangements, i.e., laterally and in the bottom.
Known in the art are the AOD-method (argon, oxygen, decarburization) and the improved AOD-(L) method (argon, oxygen, decarburization) (Lance) with a converter having a specific volume of 0.45-0.60 m.sup.3 /t steel.
The original method operates only with one or more bottom nozzles, while the improved method uses a lance for blowing in oxygen additionally from the top through the crucible opening. Also known is the CLU-(L) (Creusot Loire Uddeholm) method in which a portion of the inert gas is used for reducing the partial CO pressure by superheated steam.
In the C-OBM-S/C-BOP method (combined, oxygen, bottom, Maxhutte, stainless or combined, basic, oxygen process), bottom nozzles as well as side nozzles are used, wherein additionally hydrocarbons or inert gases or a mixture of both gases are used as cooling gas for protecting the nozzles.
In this method, two converters are used instead of an arc furnace and a converter, wherein pig iron dephosphorized in the so-called premelt converter is injected into a premelt and is subsequently filled into a premelt ladle and is then filled in the so-called refining converter where the refining process is carried out with a mixture of O.sub.2 +N.sub.2 +(Ar) and a hydrocarbon-containing protective gas with the addition of scrap, alloying agents and additives. In both crucibles, oxygen is additionally injected into the converter through a top lance.
Finally, the CMS-S method (combined, Maxhutte, stainless steelmaking) should be mentioned in which, in addition to the process gases mentioned above, added to the converter crucible are coke or carbon additions in pulverized or finely particulate form for afterburning, while simultaneously increasing the portion of solid metal charge materials and for improving the metallurgical process sequence.
Consequently, in both plant concepts, a premelt crucible in the form of an electric arc furnace and a refining crucible in the form of a converter are used, wherein temperature and energy losses as well as discharge and metal losses cannot be avoided each time the treatment unit is changed and the premelt is filled into another unit.
In order especially to reduce the chromium losses, the premelt must be reduced with FeSi/Al prior to each change of the crucible. However, in this case, calculated excessive reduction agents are required for achieving a reduction which is as complete as possible. The excess introduced in this manner must be once again treated with oxygen in the subsequent unit, i.e., a converter, after being refilled, before the decarburization without significant chromium slagging and further treatment steps are carried out.
DE 33 47 718 discloses a method of producing high-grade steels, particularly steels with high chromium contents, which utilizes the "duplex method" already discussed above. In this method, for producing steels having a high degree of purity and low gas contents in steel mills and steel foundries, the steel, after melting in an electric arc furnace or similar melting unit, is filled with the desired alloying components into a converter, is refined and deoxidized, wherein simultaneously purging with an inert gas takes place and the melt, after tapping from the converter into the ladle, is fined by purging with inert gas.
A converter having a specific reaction volume of between 0.45 and 0.8 Nm.sup.3 /t steel is used and the process gas supply during the converter treatment is controlled within the range of 0.5 to 2.0 Nm.sup.3 /t. min.
The process gas supply takes place in the converter through blow or injection openings in the bottom or near the bottom, wherein the converter opening is temporarily made smaller during the deoxidation phase and a collected slag containing a deoxidizing substance is added to the steel during tapping.
DE 195 09 285 C1 discloses a melting crucible plant with one or also two converter crucibles arranged next to each other, wherein the melting crucible is attached through support elements arranged on the circumference to inner travel rings, and wherein each of the inner travel rings is mounted by means of roller segments in an outer travel ring which rolls on travel rails in a travel groove.
During the steel production, either an injection lance attached to a lifting and pivoting device or the electrodes attached to a pivotable electrode support are lowered into the converter crucible in order to refine liquid metal charge material or to melt solid metal charge material.
EP 0 630 977 A1 discloses a converter for producing steel from solid and/or liquid charge materials, such as pig iron and/or scrap and/or sponge iron, which includes a refining crucible with refractory lining, a refining unit supplying oxygen or an oxygen-containing gas, and a heating unit.
For producing the steel, the heating unit includes at least one consumable graphite electrode and the refining unit is formed independently of the heating unit either by an oxygen injection lance or has oxygen-injecting bottom nozzles and/or side nozzles located underneath the melt bath level.
The method includes as a first method step a principal refinement of pig iron and possible a partial quantity of scrap and/or sponge iron, and subsequently as a second method step melting of additionally introduced scrap and/or sponge iron and refining possibly simultaneously or as a third method step, wherein during the first method step only the refining unit is active and in further method steps the refining unit as well as least as one graphite electrode are activated, wherein the activation is alternatingly in the case of injection of oxygen from the top and alternatingly or simultaneously in the case of injection of oxygen from below the melt bath level.
Neither the converter nor the method are suitable for producing stain-resistant, acid-resistant and heat-resistant qualities because, on the one hand, not available are the suitable devices for the injection of inert gases (Ar) with hydrocarbon-containing cooling gases and a suitable crucible concept and, on the other hand, specific method steps are not available for providing a solution for the stated object.